Shipboard navigation radars assist navigators of ships in avoiding collisions by allowing the navigators to locate land and objects (e.g., ships, buoys) beyond what can been seen from the ship itself. In early shipboard navigation radars, analog circuits were used to generate a two-dimensional image (e.g., a PPI) on a cathode ray tube (CRT) display. These analog circuits formed the image by driving the rotation of the cathode ray around the CRT in synch with the rotation of the radar antenna, by re-centering the cathode ray with each trigger of the radar transmitter (or transceiver), and by timing the sweep of the cathode ray with the radar echoes processed by the radar receiver (or transceiver). This sort of image was swept out in real time, using the persistence of phosphors in the CRT to hold the image on the display long enough to be useful in navigation.
With the advent of inexpensive television CRT displays and flat panel displays of large size, radar engineers migrated the plan position indicator (PPI) image generation away from analog circuits into digital electronic circuits and firmware. Because these CRT displays and flat panel displays are designed for television, however, they generate images using a raster scan. In other words, CRT displays and flat panel displays generate images using data formatted in Cartesian coordinates rather than data formatted in polar coordinates (azimuth angle and range) natural for radars. The adaptation of radar to raster-scanned monitors may be achieved by converting the polar formatted data (generated by radar antennas) into Cartesian coordinates (which can be displayed on raster scan CRT displays and flat panel displays).